1. Field of the Invention
The present invention relates generally to power converters, and more specifically to a primary-side-regulation power converter.
2. Description of the Related Art
A power converter is used to convert an unregulated power source to a regulated voltage or current. The power converter normally includes a transformer having a primary winding and a secondary winding to provide galvanic isolation. A switching device is generally connected to the primary winding to control energy transfer from the primary winding to the secondary winding.
FIG. 1 shows a primary-side-regulation power converter. A transformer 10 comprises a primary winding NP, a secondary winding NS and an auxiliary winding NA. Since the auxiliary winding NA and the secondary winding NS are magnetically coupled, the detection signal VDET obtained from the auxiliary winding NA will be correlated to an output voltage VO generated by the secondary winding NS via a rectifier 40 and a capacitor 45. Energy sourced from the auxiliary winding NA charges a capacitor 15 via a rectifier 12. A supply voltage VCC obtained across the capacitor 15 is utilized to power a controller 50. The controller 50 generates a driving signal SPWM to regulate an output current IO of the primary-side-regulation power converter in response to the detection signal VDET. The driving signal SPWM drives a power transistor 20 for switching the transformer 10. The transformer 10 transfers the energy of an input voltage VIN to generate the output voltage VO of the primary-side-regulation power converter. A resistor 30 is connected in series with the power transistor 20 to convert a primary-side switching current IP flowing through the power transistor 20 into a switching-current signal VIP. The switching-current signal VIP is supplied to the controller 50 for regulating the primary-side-regulation power converter. The primary-side-regulation power converter will be operated at discontinuous current mode (DCM) when the transformer 10 is fully discharged before next switching cycle starts. When the driving signal SPWM is enabled before the transformer 10 is fully discharged, the primary-side-regulation power converter will be operated at continuous current mode (CCM).
FIG. 2 shows the waveforms of the driving signal SPWM and the switching-current signal VIP. A continuous current IA represents the energy stored in the transformer 10 before next switching cycle starts. When the continuous current IA is equal to zero, the primary-side-regulation power converter is operated at DCM. Otherwise, the primary-side-regulation power converter will be operated at CCM when the continuous current IA is not equal to zero. A ramp current IC represents the energy that is further charged to the transformer 10 during an on-period TON of a switching cycle T of the driving signal SPWM. A peak current IB equals to the sum of the continuous current IA and the ramp current IC.
In order to regulate the output current IO from the primary side of the transformer 10, the continuous current IA is a parameter needed to know. Since a voltage spike will appear at the switching-current signal VIP whenever the driving signal SPWM becomes enabled, a true value of the continuous current IA will be therefore difficultly to be obtained. In conventional arts, two independent sampling circuitries are utilized to respectively sample two magnitudes of the switching-current signal VIP for calculating the continuous current IA. However, two sampling circuitries have their respective inherent operational errors, which affect the regulating precision of the primary-side-regulation power converter. Furthermore, a double-size layout space will be occupied, which results in a higher manufacturing cost for the controller 50.
Therefore, a more precise and lower cost solution for regulating the output current IO of the primary-side-regulation power converter operated at both continuous current mode and discontinuous current mode is desired by the industries.